Plugin updates do not appear to complete on some sites

[bobbingwide]

Note: This problem may explain the issue I’d noticed where plugin and theme updates didn’t appear to complete on some sites. The screen displayed the message for the first plugin but not other others that had been selected.

Originally posted by @bobbingwide in #147 (comment)

Problem

On some sites, if you select more than one plugin to update the new page displays the applying updates message but never seems to come to an end.
I don’t know if other people have experienced this problem or the situations in which this problem occurs. It may simply be a server timeout.

First thing to do is to capture some evidence. Plenty of opportunity while applying updates for WordPress 5.5.1.

It didn’t seem to be a problem updating a WordPress Multi Site installation.

[bobbingwide]

Doesn't seem to be the case. Here's a screen capture of a theme update being applied after updating oik to 4.1.0

There's supposed to be a spinner, but it's not visible.
Installed version of WordPress was 5.4.2. Version to update to was 5.5.1 en_GB version