Hello,
Did you previously downgrade your 2019 and 2022 RDS CAL through a seller, or is it installed as a "Built-In Overused" group in the 2019 license server?
In fact,before April, the 2019 CAL is compatible with both the 2016 session host and the 2019 license server. It can be installed, but it will be displayed in the "Built-In Overused" group.
If it was once downgraded now become the 2016 CAL, the 2016 CAL is not compatible with the 2019 session host; in this case, you may need to contact your seller again. We apologize as we are not trained licensing experts and license issues can be complex sometimes; for more accurate information, we recommend contacting the seller. Our focus here is more on technical issues, such as a CAL installed properly but not issued.
If it is installed as a "Built-In Overused" group, the CAL is still the 2019 version, then they are definitely compatible after April. However, RDS 2022 CAL is not compatible with 2019 license server.
Note: "Built-In Overused" group is shown when RDLS OS version is higher than the RDSH OS version and there are no RDS CALs available on the RDLS with the same level as the RDSH. It is a reporting mechanism to tell that these number of users have logged in without an appropriate CAL.
This was an indication for admins that they must install licenses. After the applicable licenses get installed, this group goes away and the number of licenses issued gets synchronized with the installed license group.
Other reference:
Upgrading your Remote Desktop Session Host | Microsoft Learn
I hope the above information helps!
Best regards,
Karlie