x11/tk/8.6: Missing headers in package?

I'm trying to build a port (netgen, port attached) that depends on Tk
and I'm trying to use version 8.6. However when building I get the
following error:

/usr/ports/pobj/netgen-6.2.2404/netgen-6.2.2404/ng/Togl2.1/togl.c:30:10: fatal error: 'tkInt.h' file not found
#include <tkInt.h> // don't need it on osx ???
^~~~~~~~~
1 error generated.
ninja: build stopped: subcommand failed.

I looked at the package lists of both Tk versions and 8.5 includes the
above mentioned header as well as the following, many of which version
8.6 does not include.

include/tk8.5/default.h
include/tk8.5/tk.h
include/tk8.5/tkDecls.h
include/tk8.5/tkInt.h
include/tk8.5/tkIntDecls.h
include/tk8.5/tkIntPlatDecls.h
include/tk8.5/tkIntXlibDecls.h
include/tk8.5/tkPlatDecls.h
include/tk8.5/tkPort.h
include/tk8.5/tkUnixDefault.h
include/tk8.5/tkUnixInt.h
include/tk8.5/tkUnixPort.h
include/tk8.5/ttkDecls.h
include/tk8.5/ttkTheme.h

Is there a specific reason for this? The Tk-8.6 package lists of other
OS's (e.g. Arch Linux[1] or FreeBSD[2]) include them as well.

I can build with version 8.5 with no issues, however this port will be
used in conjunction with cad/opencascade which depends on version 8.6.
Not sure if this would cause problems or not, but it seems silly to
install both versions if both could work with the same.

[1] https://archlinux.org/packages/extra/x86_64/tk/files/
[2] https://github.com/freebsd/freebsd-ports/blob/main/x11-toolkits/tk86/pkg-plist