pt-online-schema-change failed when creating new tablle, specified key was too long; max key length is 767 bytes issue

The version, mysql is InnoDB 5.6.10-log, pt-online-schema-change 3.1.0. table info is

email varchar(225) COLLATE utf8mb4_unicode_ci DEFAULT NULL,
uid varchar(255) COLLATE utf8mb4_unicode_ci DEFAULT NULL,
hotel_brand_id int(11) DEFAULT NULL,

UNIQUE KEY index_membership_customers_on_uid_and_hotel_brand_id (uid,hotel_brand_id),

My purpose is to add a unique index,

pt-online-schema-change --recursion-method=none --nocheck-unique-key-change h=$DATABASE_HOST,D=$DATABASE,t=$TABLE,u=$DATABASE_USER,p=$DATABASE_PASSWORD   --alter "ADD UNIQUE INDEX index_membership_customers_on_email_and_hotel_brand_id (email, hotel_brand_id)"   --$RUN --alter-foreign-keys-method=auto

then I met the unique index length issue(https://stackoverflow.com/questions/1814532/mysql-error-1071-specified-key-was-too-long-max-key-length-is-767-bytes) ,

Creating new table...
Error creating new table: DBD::mysql::db do failed: Specified key was too long; max key length is 767 bytes [for Statement "CREATE TABLE umami_prod._membership_customers_new (

It seems this was the cause

uid varchar(255) COLLATE utf8mb4_unicode_ci DEFAULT NULL,
UNIQUE KEY index_membership_customers_on_uid_and_hotel_brand_id (uid,hotel_brand_id)

so I want to resize the columns or change its character set, but whatever I got to do, it failed because pt-online-schema-change try to create a new table first, then it failed because of this index length issue in the beginning. I couldn't change anything So my question is 1. How does the current table support this utfm8 varchar(255), How does it possible?

UNIQUE KEY index_membership_customers_on_uid_and_hotel_brand_id (uid,hotel_brand_id)

2. Maybe there are table settings I missed for pt-online-schema-change 3. Anything I couldn't do, next, it is a 4M rows size table, and since pt-online-schema-change couldn't work because of the above issue, my only option is alter, to lock whole table? Thanks for your reply, any hint might help, thanks