It is only calculated if there is “time in zones” information for the activity which requires power, HR or (for runs) pace data. So @Morten_Horning_Jense your ride on the 23rd of Jan 2023 doesn’t have any power or HR data so no time in zones.
@Adrian_M_Gamble I can see quite a few of your rides have 0.00 PI. Thats because you don’t have any time in Z5 or Z6 on those. Is that what you mean by not displaying?