I am hoping someone on the forum can help. I have been working with both Humminbird and Navionics.

Yes, my units are all up to date! Yes, I have cleared all Nav Data. Yes, the card worked, unit...

To reduce the number of waypoints, trails and groups, I exported that data to a new micro SD card. Before the export, the contours for the lake I was about to go on were visible. Before doing the export, and while the unit was on, I took the Navionics card out. After completing the export, I reinserted the Navionics+ Regions card. At that point, I could no longer see the Navionics+ Regions chart card data.

Navionics sent me a new card and I updated it before inserting it in my unit. The issue continues. I can see all of the chart data on my computer, just not on my Humminbird units. I have two Helix 9 Chirp Mega SI + GPS G3N. I have tried the card in both units with the same result.

I can see the Humminbird Basemap. The issue is the same whether the unit is set to auto select the card or if I select the Navionics card directly.

Has anyone else had this issue? Looking for help!

Depends on the body of water. Have you turned on Sonar Charts to see if it has coverage and Nautical Charts doesn't?
Did you add Sonar Charts when you did the update,?

Sounds to me like your units have quit reading data from the SD card sockets. I have Helix 9's and they both will read from the cards. I don't have a clue what you might have done in the process to take them out of the loop but based on what you said it might be with the update of the humminbird software. Any chance you would have a Lakemaster card you could try? Lakemaster is more native to humminbird than navionics.
Good luck
BD