Why does waiting work around the problem?
Because it takes time for X clipboard managers to respond to selection
ownership changes and to complete reading from other programs'
selections.

Not a bug, sorry.

Thanks for the explanation, and no need to apologise!

For those of us wanting to read more on this, I dug around a bit and found some explanations

here https://github.com/jhunt/xclipd,

here https://jameshunt.us/writings/x11-clipboard-management-foibles/,

and here https://www.uninformativ.de/blog/postings/2017-04-02/0/POSTING-en.html