IMHO this type of learning can be work as long as it aligns with what the company needs. Dicking around with Rust because you think it might be useful one day doesn't count. There's a grey area for sure.
I also think that it's healthier to strive for setting+hitting ambitious outcomes, and thinking less about literal hours that you're working.
(My blog co-author wrote a post about a very closely related topic - the TLDR is that it's really a manager's job to look for ways to train their team by aligning learning areas with what the business needs. Without that alignment, you'll always have the tension that you called out where it can feel like you're not doing "real work" while learning: https://staysaasy.com/management/2020/08/01/Growth-Paths.htm...)
I also think that it's healthier to strive for setting+hitting ambitious outcomes, and thinking less about literal hours that you're working.
(My blog co-author wrote a post about a very closely related topic - the TLDR is that it's really a manager's job to look for ways to train their team by aligning learning areas with what the business needs. Without that alignment, you'll always have the tension that you called out where it can feel like you're not doing "real work" while learning: https://staysaasy.com/management/2020/08/01/Growth-Paths.htm...)