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it seems like there's some collision in IDs or somesuch

There's a lot going on in this model. A skeleton part studio, pieces built from various derives from that, etc. This particular piece was modeled left half first, then mirrored to the right, then the face at the bottom etc were selected for fillets, and now I went to make this new fillet for the pair of recesses.
So… the interesting bit is that when I select the first edge on the right side of the left recess… it fillets that edge and the corresponding edge mirrored on the other side. But when I select a second edge (the left side of the left recess) it does not fillet the corresponding edge on the right hand mirrored half of the part.
As a detail, directions above were in reference to the image though the screenshot was taken from the rear of the part.
Perhaps some combination of the mirroring and then a single tangent-edge chain being filleted is resulting in some sort of edge ID collision? Meh, just wild guessing.https://cad.onshape.com/documents/e4d19ddefeea25c0af61e545/v/97bdf1db05099ca55d04ac9d/e/2b21b2351010c92d78ae6ef5?renderMode=0&uiState=67cf73e950c8244d945e8497
Comments
Try turning off tangent propagation?
Why are you filleting after the mirror?
This is one reason why to include as much as possible before the mirror.
My guess would not be ID collision, but that the fillet is finding it hard to come up with a proper solution in the corner and it's on the brink of failing there…
or as S1mon suggests, it's trying to include a tangent edge that's not filletable and attemping to fillet…
BTW, if you can allow some fillets on the vertical edges of the pocket, you'll get a better fillet result in the corners i.m.h.o.
I agree… not a bug. Sorry. Turning off tangent propagation avoids it. I guess the prior fillet that aligned with the edge of the recess resulted in me selecting the edge of that fillet while I was thinking about the edge of the recess. it then tangent propagated to the other side with no geometry change on the existing larger radius fillets in the middle.
Because various little reasons that I had not dealt with since the core pieces etc are still moving around enough that I didn't finalize the fillet 'design'.
:]
I usually would add fillets inside the recess as you show but so far had hesitated for the sake of simplicity of the other part that fits in there. It's a stack of 4x 2mm x 15mm x 140mm 'spring bars'. I may yet add the fillets you suggested. In cases where I don't have those I use the smooth fillet corners option to get half of that effect, though it isn't applying in this case right now. Mostly, I have too many fillets running into each other right there and should get a little more (than zero/negative) separation.Thanks for the pointers and for immediately calling out on my misallocation of blame for this merely unexpected behavior.