Sometimes setting ‘flat export’ in SquareLine Studio Project Settings solves Arduino-IDE build problems (as suggested here for example: 'ui_Button1' undeclared - Arduino compiler throws this error after renaming buttons - #4 by JGB )
In this case it seemed not to solve the problem.
Arduino IDE is made with simplicity in mind from user perspective, but at the same time this means it expects specific folders and files in place (‘src’/‘libraries’) and with certain extensions (‘.cpp’,‘.h’), to do things fairly automatically, and when problems like this occur it’s needed to check what it does under the hood. I found a forum topic at stackoverflow about the possible quirks and solutions, maybe a good starting point to understad Arduino IDE’s build-chain: c++ - How do I add cpp file to an arduino project? - Stack Overflow
Our investigation will not end here, but if seasoned Arduino-IDE users have further tips about the possible issue it’s still welcome…Maybe a look at LVGL-forum (this is more of an LVGL-related question) will reveal some solutions, e.g. with keywords ‘arduino montserrat’ I found a testing for this case (but no solution): 'lv_font_montserrat_48' was not declared - #3 by kisvegabor - General discussion - LVGL Forum .
While not obvious, I guess the Arduino projects can be built in commandline too with Arduino-SDK, and there are directions for CMake builds too which are probably more efficient, flexible and more transparent than Arduino-IDE’s simple GUI…