It seems like that the MBOX export is limited to 505 mails per folder. There is no error message about it. I experienced that behaviour first will trying to import the mbox-files to anohter mailer. After that I created a test folder. One simple mail multiplied. I used a second help folder for that and did copy and paste per drag & drop between the folders. I had three test folders after that with 505, 506 and 1240 mails. Each mail has 39 lines in its source including the blank line at the end. This was the indicator to sureley count the number of exported mails. The export from all three folders have the same size in KB, the same number of lines (exact 505 x 39). There are also other bugs related to this one.
Related to #4308 "Exported MBOX file emtpy in special cases" https://www.thewildbeast.co.uk/claws-mail/bugzilla/show_bug.cgi?id=4308
works for me under linux. Not able to test with windows 10, so leaving open but reverting back to "Claws Mail (Windows)" and "Windows 10" until it can be verified there.
How do you verified that "it works" for you? Did you explicite count the mails in the exported MBOX file? There is no error message when there are more then 505 mails! The bug is silent.
I got curious if this is actually a bug and tested it on my system too. It just works. The number of exported emails is correct and it does not stop at 505. I tested it by exporting first (then I checked the number of lines starting with "From:") and imported again. The imported number of mails is correct here. Tested on a linux system though.
Please can you be more specific about your testing methode. You used e.g. a Python script for checking? When you realy checked only the start of lines is not enough. Because in an mbox mails contain more then one "From" lines. You have to check for "\nFrom " (Attention! Without the ':'). Side question to the core devs: Which mbox-dialict is really impelemented by Claws? To my side: I still can reproduce also with different folders. I used a real life (not test date) folder with 1309 mails. I imported it into Thunderbird and got a folder with 505 mails. I tried the other way arround und imported that mbox file back into Claws (to a different empty folder). Funny! Claws tells me "Error: Mbox-Import fault" (translated by me; Original German: "Fehler: Mbox-Import schlug fehl."). I used Notepadd++ to search for "\nForm ". Exactly 504 counts - the first mail does not start with a blank line. So in the end there are 505 mails in it.
I can not reproduce this on Linux Claws 3.17.3 from Debian 10 repository. Export from Claws, Import into Claws and Thunderbird works fine as expected. Looks like a Windows specific problem.
I can confirm this must be Windows specific. Tried today with a mail account on another machine and another claws instance. The export with Windows Claws fails as described. I started a VM with Debian 10 on that Windows machine and used the same claws-mail and account-name folder with the Claws instance on Linux. On Linux it works fine.