Index scan when more than 35 correlated subqueries are used with default cardinality estimation

Recently, we updated the compatibility level of our SQL Server from 2012 to 2016, but after updating the compatibility level we ran into performance issues when a lot of sub queries are used. Especially when more than 35 subqueries are used. Here is a query with which I can reproduce it:

SELECT 
  [PK_R_ASSEMBLYCOSTING],
  (SELECT SUM([PRICEEXMARKUP]) FROM [R_ASSEMBLYCOSTINGITEM] WHERE [FK_ASSEMBLYCOSTING] = [PK_R_ASSEMBLYCOSTING]),
  (SELECT SUM([PRICEEXMARKUP]) FROM [R_ASSEMBLYCOSTINGITEM] WHERE [FK_ASSEMBLYCOSTING] = [PK_R_ASSEMBLYCOSTING]),
  (SELECT SUM([PRICEEXMARKUP]) FROM [R_ASSEMBLYCOSTINGITEM] WHERE [FK_ASSEMBLYCOSTING] = [PK_R_ASSEMBLYCOSTING]),
  (SELECT SUM([PRICEEXMARKUP]) FROM [R_ASSEMBLYCOSTINGITEM] WHERE [FK_ASSEMBLYCOSTING] = [PK_R_ASSEMBLYCOSTING]),
  (SELECT SUM([PRICEEXMARKUP]) FROM [R_ASSEMBLYCOSTINGITEM] WHERE [FK_ASSEMBLYCOSTING] = [PK_R_ASSEMBLYCOSTING]),
  (SELECT SUM([PRICEEXMARKUP]) FROM [R_ASSEMBLYCOSTINGITEM] WHERE [FK_ASSEMBLYCOSTING] = [PK_R_ASSEMBLYCOSTING]),
  (SELECT SUM([PRICEEXMARKUP]) FROM [R_ASSEMBLYCOSTINGITEM] WHERE [FK_ASSEMBLYCOSTING] = [PK_R_ASSEMBLYCOSTING]),
  (SELECT SUM([PRICEEXMARKUP]) FROM [R_ASSEMBLYCOSTINGITEM] WHERE [FK_ASSEMBLYCOSTING] = [PK_R_ASSEMBLYCOSTING]),
  (SELECT SUM([PRICEEXMARKUP]) FROM [R_ASSEMBLYCOSTINGITEM] WHERE [FK_ASSEMBLYCOSTING] = [PK_R_ASSEMBLYCOSTING]),
  (SELECT SUM([PRICEEXMARKUP]) FROM [R_ASSEMBLYCOSTINGITEM] WHERE [FK_ASSEMBLYCOSTING] = [PK_R_ASSEMBLYCOSTING]),
  (SELECT SUM([PRICEEXMARKUP]) FROM [R_ASSEMBLYCOSTINGITEM] WHERE [FK_ASSEMBLYCOSTING] = [PK_R_ASSEMBLYCOSTING]),
  (SELECT SUM([PRICEEXMARKUP]) FROM [R_ASSEMBLYCOSTINGITEM] WHERE [FK_ASSEMBLYCOSTING] = [PK_R_ASSEMBLYCOSTING]),
  (SELECT SUM([PRICEEXMARKUP]) FROM [R_ASSEMBLYCOSTINGITEM] WHERE [FK_ASSEMBLYCOSTING] = [PK_R_ASSEMBLYCOSTING]),
  (SELECT SUM([PRICEEXMARKUP]) FROM [R_ASSEMBLYCOSTINGITEM] WHERE [FK_ASSEMBLYCOSTING] = [PK_R_ASSEMBLYCOSTING]),
  (SELECT SUM([PRICEEXMARKUP]) FROM [R_ASSEMBLYCOSTINGITEM] WHERE [FK_ASSEMBLYCOSTING] = [PK_R_ASSEMBLYCOSTING]),
  (SELECT SUM([PRICEEXMARKUP]) FROM [R_ASSEMBLYCOSTINGITEM] WHERE [FK_ASSEMBLYCOSTING] = [PK_R_ASSEMBLYCOSTING]),
  (SELECT SUM([PRICEEXMARKUP]) FROM [R_ASSEMBLYCOSTINGITEM] WHERE [FK_ASSEMBLYCOSTING] = [PK_R_ASSEMBLYCOSTING]),
  (SELECT SUM([PRICEEXMARKUP]) FROM [R_ASSEMBLYCOSTINGITEM] WHERE [FK_ASSEMBLYCOSTING] = [PK_R_ASSEMBLYCOSTING]),
  (SELECT SUM([PRICEEXMARKUP]) FROM [R_ASSEMBLYCOSTINGITEM] WHERE [FK_ASSEMBLYCOSTING] = [PK_R_ASSEMBLYCOSTING]),
  (SELECT SUM([PRICEEXMARKUP]) FROM [R_ASSEMBLYCOSTINGITEM] WHERE [FK_ASSEMBLYCOSTING] = [PK_R_ASSEMBLYCOSTING]),
  (SELECT SUM([PRICEEXMARKUP]) FROM [R_ASSEMBLYCOSTINGITEM] WHERE [FK_ASSEMBLYCOSTING] = [PK_R_ASSEMBLYCOSTING]),
  (SELECT SUM([PRICEEXMARKUP]) FROM [R_ASSEMBLYCOSTINGITEM] WHERE [FK_ASSEMBLYCOSTING] = [PK_R_ASSEMBLYCOSTING]),
  (SELECT SUM([PRICEEXMARKUP]) FROM [R_ASSEMBLYCOSTINGITEM] WHERE [FK_ASSEMBLYCOSTING] = [PK_R_ASSEMBLYCOSTING]),
  (SELECT SUM([PRICEEXMARKUP]) FROM [R_ASSEMBLYCOSTINGITEM] WHERE [FK_ASSEMBLYCOSTING] = [PK_R_ASSEMBLYCOSTING]),
  (SELECT SUM([PRICEEXMARKUP]) FROM [R_ASSEMBLYCOSTINGITEM] WHERE [FK_ASSEMBLYCOSTING] = [PK_R_ASSEMBLYCOSTING]),
  (SELECT SUM([PRICEEXMARKUP]) FROM [R_ASSEMBLYCOSTINGITEM] WHERE [FK_ASSEMBLYCOSTING] = [PK_R_ASSEMBLYCOSTING]),
  (SELECT SUM([PRICEEXMARKUP]) FROM [R_ASSEMBLYCOSTINGITEM] WHERE [FK_ASSEMBLYCOSTING] = [PK_R_ASSEMBLYCOSTING]),
  (SELECT SUM([PRICEEXMARKUP]) FROM [R_ASSEMBLYCOSTINGITEM] WHERE [FK_ASSEMBLYCOSTING] = [PK_R_ASSEMBLYCOSTING]),
  (SELECT SUM([PRICEEXMARKUP]) FROM [R_ASSEMBLYCOSTINGITEM] WHERE [FK_ASSEMBLYCOSTING] = [PK_R_ASSEMBLYCOSTING]),
  (SELECT SUM([PRICEEXMARKUP]) FROM [R_ASSEMBLYCOSTINGITEM] WHERE [FK_ASSEMBLYCOSTING] = [PK_R_ASSEMBLYCOSTING]),
  (SELECT SUM([PRICEEXMARKUP]) FROM [R_ASSEMBLYCOSTINGITEM] WHERE [FK_ASSEMBLYCOSTING] = [PK_R_ASSEMBLYCOSTING]),
  (SELECT SUM([PRICEEXMARKUP]) FROM [R_ASSEMBLYCOSTINGITEM] WHERE [FK_ASSEMBLYCOSTING] = [PK_R_ASSEMBLYCOSTING]),
  (SELECT SUM([PRICEEXMARKUP]) FROM [R_ASSEMBLYCOSTINGITEM] WHERE [FK_ASSEMBLYCOSTING] = [PK_R_ASSEMBLYCOSTING]),
  (SELECT SUM([PRICEEXMARKUP]) FROM [R_ASSEMBLYCOSTINGITEM] WHERE [FK_ASSEMBLYCOSTING] = [PK_R_ASSEMBLYCOSTING]),
  (SELECT SUM([PRICEEXMARKUP]) FROM [R_ASSEMBLYCOSTINGITEM] WHERE [FK_ASSEMBLYCOSTING] = [PK_R_ASSEMBLYCOSTING]),
  (SELECT SUM([PRICEEXMARKUP]) FROM [R_ASSEMBLYCOSTINGITEM] WHERE [FK_ASSEMBLYCOSTING] = [PK_R_ASSEMBLYCOSTING]),
  [PK_R_ASSEMBLYCOSTING]
FROM [R_ASSEMBLYCOSTING]
WHERE [FK_ASSEMBLY] = 309961

When there are less than 35 subqueries, the query plan shows it is using index seeks for the subqueries: But for every additional subquery over 35 an index scan is used: Does anybody have any explanation why this happens? If Legacy Cardinality Estimation is enabled, the query is fast and doesn't have this issue, but we want to disable that. I already tried to rebuild indexes and update the statistics but that doesn't make any difference.