Performed the update and at least imapsync now doesn't report an issue with searching, due a midb issue any more:
Checking SEARCH ALL works on both accounts. To avoid that check, use --nochecknoabletosearch
Host1: checking if SEARCH ALL works on INBOX
Host1: folder [INBOX] has 26912 messages mentioned by SELECT
Host1: folder [INBOX] has 26912 messages found by SEARCH ALL
Host1: folder [INBOX] has the same messages count (26912) by SELECT and SEARCH ALL
Host2: checking if SEARCH ALL works on INBOX
Host2: folder [INBOX] has 0 messages mentioned by SELECT
Host2: folder [INBOX] has 0 messages found by SEARCH ALL
Host2: folder [INBOX] has the same messages count (0) by SELECT and SEARCH ALL
Good! SEARCH ALL works on both accounts.
I also raised the midb_table_size to 50000 as mentioned on the other thread and now I am running grommunio-imapsync on a newly create mailbox.