I couldn't reproduce. I wonder if there's maybe some caching issue.

I'd need a precise reproduction case - i.e. a series of screenshots showing:
initial state
first changed state (before saving)
first changed state (after saving)
second changed state (before saving)
second changed state (after saving) [with bug showing here]

What it might be is that reorder is not defined if you set multiple things to the same order.

For example, if you set forum# orders like:
<code>#1=1,#2=1,#3=2</code>

You may get like
<code> #1=1,#2=2,#3=3</code>

But it depends on other factors, you could get:
<code>#1=1,#2=3,#3=2</code>

Or
<code>#1=3,#2=1,#3=2</code>

Ergh, I could not get Mantis to not parse those numbers, oh well

Okay, I will try and put some screenshots together. I made sure the id's weren't clashing every time so maybe it is the cache. Will get back to you on this...