Heart rate reading error

As of the last week or so, my fenix 6 seems to be significantly under-reading my heart rate for the first 10 mins or so of a work out, before then reverting to a 'correct' reading. See example below from 20 min indoor bike ride last night. And I'm seeing the same for other activities e.g. runs. Seems to hover below 120bpm for a while, despite pushing it, and then suddenly shooting up to 150 to160. I've used the watch a lot for running, know pretty much were I expect my heart rate to be, and this has never happened previously.

Anyone else seen this? Software glitch? 

Thanks