Dataset thumbnail not applied in some cases

[jp-tosca]

What steps does it take to reproduce the issue?

in #10220 was reported the thumbnails were not working properly after 6.0 and 3 bugs were identified during the testing and 2 of them were addressed on #10258. The remaining issue was initially identified by @landreev on this comment but after some testing, I noticed that this would also happen in some cases where the Dataset had already images, more info here it was clear to me that this may require to look a bit deeper on the code @qqmyers provided information here that can be useful to solve this bug.

• When does this issue occur?
  Still not clear, initially was suspected that this would happen only when a dataset had no other images attached but I was able to reproduce this on Datasets with images.

• Which page(s) does it occurs on?
  Dataset result page.

• What happens?
  A custom thumbnail will not show up.

• To whom does it occur (all users, curators, superusers)?
  Everyone

• What did you expect to happen?
  A dataset should show the custom thumbnail when defined.

Which version of Dataverse are you using?
This bug was discovered during a report after 6.1

Any related open or closed issues to this bug report?
#10220
#10258

Screenshots: