I'm having problems with arpeggiator rate as a mod matrix destination on my microfreak as well. Firmware version is 4.0. I was following along with this tutorial when I noticed the problem:
https://youtu.be/vkudcaEQr9w?t=903I've set my sync mode to internal and verified the arp is not in sync mode (display reads "Tempo" after pressing the Rate/Swing button). As in that video above, I'm using an init patch modified to have the Key/Arp set to modify the arp rate (as assignable 1) with a depth of -100. I'll put the arp in hold mode, add a few notes, and extend the arp range to 3 octaves to ensure the modulation of arp rate should be pretty significant. Instead, it just holds a steady temp.
If I then try manually changing the rate, the tempo will change significantly and then hold there. For example say I was at 120.0 bpm and I increase the tempo to 120.1 bpm. The actual tempo will then jump to something like 90 bpm. The display will still indicate 120.01 bpm, but the actual arp is playing significantly slower than it was at 120.0 bpm. The change to the effective bpm is not consistent. I'm guessing it is related to the arp note that is playing when the new tempo is applied. Maybe the key/arp modulation is being determined when BPM updates from the knob, but is not being applied from subsequent notes from the arp.
aguadopd - it sounds like this was working for you last year. Do you know if you are on the same FW version now as you were back then? If you've updated, I'm curious to see if this still works for you. It'd be useful to know if the behavior is only on certain units, or only on specific firmware versions.