Gerald,
When in arbitrary precision mode, rotating the image by exactly +-90 degrees causes the rotation angle in the TAB-window to be shown as (-)57.296 degrees (image calculation, however, remains correct at +-90 degrees).
Saving the current settings as a par gets the (-)57.296 degrees rotation to be written to the file (that's how I discovered this behaviour).
Changing the par's "center-mag=" by hand to (-)90 degrees and loading it into Fractint results in the same (correct calculation, but wrong TAB display), but now use of any other user interface screen (i.e. "x") causes Fractint to continue with the (-)57.296 degree angle.
Fixed it. Committed the changes to the repository. The problem was introduced after version 19.0 and prior to version 19.6. The result you end up seeing is caused because a math error occurs at a call to atan2(), which we catch and provide an output of 1.0. This is interpreted as 1 radian or 57.296 degrees. Jonathan