Update Manager shows fewer updates on first load than after manual refresh

Describe the bug
Update Manager shows fewer updates on first load than after manual refresh.

To Reproduce

1. Open Update Manager
2. Note the number of updates shown (4 updates appeared)
3. Click Refresh
4. More updates appear that were not visible before (5 updates appeared)

Expected behavior
All available updates should be shown on first load without needing to manually refresh.

Distribution:

• Linux Mint

Software version:
mintupdate 7.1.4

Logs :
First load (4 updates shown):
2026.03.21@22:27 ++ Launching Update Manager
2026.03.21@22:27 ++ Checking for updates
2026.03.21@22:27 ++ Found 4 software updates
2026.03.21@22:27 ++ Refresh finished

After manual Refresh (5 updates shown):
2026.03.21@22:28 ++ Refreshing cache
2026.03.21@22:28 ++ Refreshing cache for Cinnamon updates
2026.03.21@22:28 ++ Refreshing cache for Flatpak updates
2026.03.21@22:29 ++ Found 5 software updates
2026.03.21@22:29 ++ Refresh finished

Additional context
On first load, the cache is NOT refreshed before checking for updates.
On manual refresh, the cache IS refreshed first, revealing additional updates.
The fix should ensure the cache is refreshed on first load.

• Linux Mint: 22.3
• Cinnamon: 6.6.7

Start at the Update Manager first-load path and compare it with the manual Refresh path. Trace when the cache, including Cinnamon and Flatpak updates, is refreshed before checking for updates. Done means the initial load shows the same complete update count as a subsequent manual refresh.