Can't move 'folders' on Apple 'Mail' using Apple M3 Max with macOS Sonoma 14.5

Started using macOS 'Mail' (migrated from PostBox 7.0) on my new ARM Apple M3 Max with macOS Sonoma 14.5.

Worked fine for the first month or so & now found one of my folders 'outside' my 'Inbox'. I tried to move or drag my folder to the correct location and it "won't move".

Folder 'Tech' outside 'Inbox':

If I try to move 'Tech' to another folder it 'bounces' back if I try & put it inside 'Inbox'

Can only move it to a new folder say inside '_me':

TIA.