Thank you @BevOutside for raising the question and @DanK for sharing your experience.
Just to let you know what is likely happening in the background, I should mention that the message you’re seeing refers to the “default” threshold. The “default” activity is an activity other than running, swimming, biking, rowing, S&C, walking, yoga. An activity that, somewhat we could not recognise automatically from the device you used to record it.
For instance, @BevOutside, your HR “default” threshold comes from an activity you recorded on January 18th and it was classified as “other” by Athletica.
This is my best understanding of the issue. For you being in a rowing plan and doing also cycling activities, the “default” threshold will not have an impact on the prescribed sessions.
I hope it makes sense.