I looked into this a while back and found two separate mechanisms. One uses cookies stored on the user's computer and these are vulnerable to browser settings, browser updates, ad blockers, and anti-virus software etc. In other words getting logged off mysteriously is a feature of your computer's set-up or confusion due to you using more than one computer.
Second, the forum is hosted by Microsoft IIS/8.5. IIS has a server-side login remembering mechanism that can override cookies, typically to free-up server resources when they run short. Very like dad running round after teenagers switching off the lights because the yoof don't worry about paying the bill! If too many users are logged on the system will follow an algorithm to decide who gets dumped, usually it is people who have been inactive for the longest time.
The Model Engineering forum isn't the only website hosted on this instance of IIS. MyTimeMedia publishes many magazines and several of them have forums suspiciously like this one:
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Quite likely they all share the same IIS. Therefore it's possible Model Engineer logins get dropped because the other forums are busy and need more space.
Can I recommend NOT staying logged in? Better discipline is to logout as soon as you've finished no matter what. Leaving a terminal logged on is sloppy lazy security and it develops into a bad habit: it increases the risk you will wander off with your Bank Account left open for your wife to find how much that Cowells really cost…
Dave
Edited By SillyOldDuffer on 23/02/2019 18:01:02