Moving Mail - Mac Mail constant crashing.

I do think this is a bug... it has happened in Sierra, High Sierra and Big Sur.

I have many emails.... and when I do a search and want to move them to a new folder Mac Mail very often crashes. It caused me to look for another email client but none where as good as Mac Mail.

I totally wiped Mac Mail, removed all preferences and started again from fresh... and when I say 'wiped' I mean wiped! I spent ages and money making sure no trace of the old Mac mail was there... then started again. but to no avail.

I did all the usual rebuilt mailboxes etc,.