SQL Server AlwaysOn cluster having issues seeding large availability database creating strange behavior
Please have a look at the links given below. They all talk about similar situations, though not identical, and can help answer a few of your questions.

Many BACKGROUND Threads with Command VDI_CLIENT_WORKER
SQL Server 2016 AlwaysOn VDI_CLIENT_WORKER command
VDI_CLIENT_OTHER wait type

Apart from this you might want to check the following -

Check the SQL Server error log for any related messages - xp_readerrorlog
Check if the log movement is &quot;Suspended&quot; for the databases that show as not synchronized on the secondary replica? - If yes, then you will need to &quot;Resume&quot; the log movement.
You might want to remove these databases (the ones that are not synchronizing) from the AG and then add them again.