The bug wasn't introduced in MorphOS 3.13: It has been there for nearly 20 years (all versions of MorphOS native ixemul.library).

What happened is a bit more complicated:
1. Unbeknownst there were two bugs: One in MorphOS itself and one in ixemul.library
2. Bug in MorphOS itself was fixed for 3.13
3. Fixing the bug in MorphOS 3.13 made the bug in ixemul.library visible

In short: The bug in MorphOS hid the bug in ixemul.library.

I'm hoping a new ixemul.library will be released shortly. Meanwhile this issue is easy to workaround by either not using ixemul.library in native ports (this is highly recommended) or by using fesetround(FE_TONEAREST); as first thing in main of the ixemul app (there are no ill side effects from this, and the ixemul app will continue to work as before even with fixed ixemul.library).