if you can reproduce error and tell us what it is I may take a look

Sfic was not added to the list at https://wikitech.wikimedia.org/wiki/Nova_Resource:Tools when subsequent users were added. Maybe some overzealous caching?

Yes they were: https://wikitech.wikimedia.org/w/index.php?title=Nova_Resource:Tools&diff=467237&oldid=463419

Does this happen every time you add multiple people? I successfully submitted the form for deployment prep with Tim Landscheidt,Merlijn van Deen
(Although the success messages looked a bit weird, https://gerrit.wikimedia.org/r/#/c/286310/ to fix that)

I added the two users in two separate browser tabs, with only one username in the input field. (I wasn't aware that it accepted multiple users, and I would need to rewrite my tl-wikitech-work-on-requests function to take advantage of that :-)).

Regarding Sfic now appearing in the list at https://wikitech.wikimedia.org/wiki/Nova_Resource:Tools: Immediately before I submitted my comment here this morning, I went to https://wikitech.wikimedia.org/wiki/Nova_Resource:Tools, clicked on "[Expand]" for "Members" and searched for "Sfic" to no avail. I did not look at or search the page source.

So maybe the error lies with Semantic MediaWiki processing the page source to database data to display data? IIRC there was a similar problem with the Bastion project where updating the list of members triggered a time-out and you (?) solved that by disabling the members list for Bastion. For Toolforge, such a solution would be suboptimal.

Honestly I'm tempted to decline this in favour of migrating to Horizon in T91988 instead (if I understand the scope of that task correctly)