Ok, in that case Logic opened last save you did. If you are sure that you saved the project after tweaking transients, then your edits must be there.
You did your edits in the file editor, and Transient Editing Mode was enabled? If you mean that markers disappeared, then try what happens if you use the + button in audio file editor, when Transient Editing Mode is enabled.
Retina 4K 21.5-inch, 2019 3,6 Ghz i3, 32 GB RAM, MacOS Catalina 10.15.7 – Logic Pro 10.6.3MacBook Air
13-inch, 2017 1,8 Ghz i5, 8 GB RAM, MacOS Catalina 10.15.7 – Logic Pro 10.6.3
Presonus Studio 26c
Novation Launchkey MK3https://soundcloud.com/user-188393460