For example:
proj4.defs("EPSG:5514", {.....}); where {....} is the PROJJSON or WKT2 string for 5514.
proj4("EPSG:5514", {x: lon y: lat});

get error:- "Could not get projection name from: EPSG:5514"

The same error happens for any PROJJSON (or WKT2) with property "conversion.method.name" containing any of these variants of Krovak:
ID Name
1041 Krovak (North Orientated)
1042 Krovak Modified
1043 Krovak Modified (North Orientated)

CAUSE:
In krovak.js at line 100: var names = ['Krovak', 'krovak'];
It's looking only for those values, however there are other variants as listed above, which will get missed.