After looking into the database (serialized_object table), the report seems to contain org.openmrs.User$HibernateProxy$ODcBnusu somewhere inside the column “serialized_data”
A snippet of this can be seen bellow:
...
<name>HSU monthly days of admission</name>
<description>HSU monthly days of admission</description>
<creator class="org.openmrs.User$HibernateProxy$ODcBnusu" id="2" uuid="A4F30A1B-5EB9-11DF-A648-37A07F9C90FB"/>
<dateCreated id="3">2021-04-15 07:56:57 UTC</dateCreated>
<changedBy id="4" uuid="1c3db49d-440a-11e6-a65c-00e04c680037"/>
....
The reports that disappear seem to share a common trait: their serialized report definitions’ <creator/> contains an Hibernate proxy class name. This is very similar to what what was happening back in 2015 and described on that other thread.
Back then it was because of Javaassist names, eg. "org.openmrs.User_$$_jvst6de_41".
Now it’s because of Hibernate proxy names, eg. "org.openmrs.User$HibernateProxy$ODcBnusu".