Odd behaviour creating / editing template variables - EE 4.2.1

Noticed that I’m getting a lot of errors trying to create / edit template variables.

Creating a new variable or trying to edit an existing from CP gives this error on saving

file_put_contents(): Exclusive locks are not supported for this stream
ee/EllisLab/ExpressionEngine/Library/Filesystem/Filesystem.php, line 101

Severity: E_WARNING

Doesn’t happen all the time, so if you try enough times eventually the action works.

Not sure what is causing - AFAIK nothing is connecting to the file store concurrently with this activity (which is solely browser based).

Grateful for any guidance on what to do to avoid the issue.