OTvar has different shapes as static exports

I have a typeface with two axes (4 masters) and no intermediate masters or bracket layers.
When exporting it as an OTvar I get different results in the interpolations of some glyphs, particularly the /M, leading to distorted shapes.
Examining the file in Samsa, the number of delta sets varies from glyph to glyph, with the /M having five delta sets (numbered 0 to 5).
Is there any reason why a 2-axis interpolation without intermediate masters requires more than 2 delta sets?
Are these deviations from the static interpolation a bug, or is it a limitation with the OTvar format?
I also have one glyph where the bold condensed extreme (axis 0=1.0, axis 1=0.0) does not match the master at all; the weight axis increases the weight up to about 75% and then decreases it back again(!) This must be a bug.
Unfortunately I can’t share the file on the forum as it’s unpublished client work.

For four masters, I expect three sets of deltas.
Is your master setup perfectly rectangular?

Yes. Weight varies from 96 to 176; width from 440 to 1000 (Glyphs units).
Silly me, of course three deltas. But the number of delta sets seems to vary from glyph to glyph.
By the way, I have tried the latest stable release as well as the latest cutting edge release and the result is the same.

Can you send me the .glyphs file?

Let me check, I should be able to do this. To which address? support@glyphsapp.com?
Thanks in advance!

I’ve sent you the files now to the above address. Thanks a lot!

Those glyphs have brace layers. They are adding a set of deltas, each.

Thanks, Georg. I had overseen the brace layers.
I have now had a closer look at the file and I seem to have found out what was causing the problem. The brace layers were pointing outside of the design space and I can only guess that Glyphs was ignoring them when generating the static exports (a warning would have been helpful), but not when generating the OTvar, and presumably the spec requires web browsers and other rendering engines to respect these deltas, even if they are outside the ranges of one or more axes.
At any rate, now that I have moved the brace values into the design space, the OTvar seems to be giving the same results as the static fonts.