Modi is a complex indic script used to write Marathi. It was predominantly used in between 1200s to 1940s in India. It was included in Unicode in 2014. I created a XKB keyboard layout, (Modi KaGaPa Phonetic) to type it on Linux platform.
I noticed that Qt apps cannot handle this script. Whenever I try to input it in any Qt text editors (like FeatherPad) or other UI elements like text fields, it is not rendered at all. It seems that Qt is not understanding input of Modi. But, I can copy-paste Modi from other sources into these apps, and it renders fine.
For verification of this issue, I tried it in apps using Qt 5.11.3 (older version) on my MX 19.4 machine. They suffer from this issue. However new versions also have this issue. (I tried it it in Scribus 1.5.7svn, using Qt 5.14.2 and it also have same issue.)
As it is a XKB layout, it should probably not have confliction with Qt. But I could not exactly figure out why it is not working with Qt. All GTK+ apps work fine.
I have not tried on other platforms.
My keyboard layout is available here : https://gitlab.freedesktop.org/xkeyboard-config/xkeyboard-config/-/blob/master/symbols/in#L2512-2579