Recenty i installed Mulch System Webview & Mulch Browser from fdroid. After that i visited on Tutanota and the page shows that Webview is Outdated. But the Tutanota App (apk) is working perfectly.
You can’t just install a webview apk and expect it to automatically be set as the webview.
Go into Developer Options and check what your webview is. More than likely it’s the one that came with your ROM (com.android.webview if on custom ROM like LineageOS).
There are various methods to get Mulch WebView working. Magisk modules or manual placement of overlay file via TWRP or adb maybe.
As @marcdw1289 said, you can’t just change webview as there is a list of allowed apps in the system.
However for standalone Mulch browser tutanota.com won’t load unless you enable the JIT for the website via the per-site settings icon in the URL bar as it needs WebAssembly to function.
I have Mulch WebView on all of my ROMs and a GSI from Oreo to A11. One can use a Magisk module (a couple/few are available) or use manual methods.
In the case of my Oreo multiboot device (no Magisk on those ROMs) it was a simple matter of manually placing the overlay file where it needed to be (/vendor/overlay/ for example). Used file manager in TWRP. Install Mulch webview and I was set.
Add the DivestOS repo to your favorite F-Droid client or use FFUpdater for updates.