I had the same issue after upgrading my MBP 2019 (Intel) to Monterey. My external Samsung C34J79x monitor was no longer charging the laptop via the TB3 connection and was also not being recognised in clamshell mode (laptop would just turn off as it thought it was being sent to sleep). The fix was to change the display settings of the external monitor to 60Hz. However, this is not ideal as I prefer the refresh rate and feel of the 100Hz. A fix for this issue would be highly appreciated Apple, it's a shame to have gone backwards in this new OS and to also not be able to use my monitor to its fullest capabilities!