GnuRadio: Clock Recovery MM: imu out of bounds
The M&amp;M clock recovery block has a bug in it, such that it can't reliably handle more than 8 samples per symbol without inadvertently dropping samples.  That interpolating resampler alignment bug may also be responsible for generating mu values outside of [0.0, 1.0].

I'd strongly recommend, downsampling the input from 2.5ksamples/symbol down to 6 or 8 samples /symbol before the M&amp;M block.