There are queue files in <Install Dir>\hub\q\<queue name>\*.sds. This sds's file sequence starts from 0 and will increase when max size reaches (default 100MB).

When hub probe is recycled, queue file sequence is reset to 0, and file size starts from 0 byte. Therefore, old queue messages are deleted.
 

hub probe 5.82 or below was using old format queue files which name start from q_<queue name>.sds in <Install Dir>\hub\q\ directory.
When there is old format queue file, hub 7.x creates new format queue file without checking existing one. Therefore, new 0.sds will be created and old queue messages are deleted. 

For example, suppose there are following files. 
<Install Dir>\hub\q\q_nas.sds   ========> This is old format file
<Install Dir>\hub\q\nas\3.sds   ========> This is new format file. And, current file sequence is 3.
After recycle of hub probe, the new format file is reset to 0. 
<Install Dir>\hub\q\nas\0.sds
 

Upgrade hub probe to 7.x from 5.82 or below.
 

After upgrade hub probe to 7.x, please make sure there is no old format file q_<queue name>.sds in <Install Dir>\hub\q\ directory.
To avoid this, please follow instruction below. 
1. Stop Nimbus 
2. delete q_<queue name>.sds from <Install Dir>\hub\q\
3. Start Nimbus
 

About Queue message
CA Unified Infrastructure Management uses subscribe mechanism. Each Robot communicates with hub probes with queue messages. The queue messages are stored in following files.
5.82 or below

<Install Dir>\hub\q\q_<queue name>.sds
7.x or above
<Install Dir>\hub\q\<queue name>\*.sds
For further description regarding the architecture, please read CA Infrastructure Management - 8.4 CA UIM Architecture Overview.