bizarre and inconsistent behavior in batch script

I have a file called tokens.txt. It is a utf-8 text file that contains a number for lines with name=value
It is used extensively in a script I have written and works 99 times out of 100. Every now and then I come across an issue I haven't been able to solve in the last 3 years.

values are like;

Code:

tanya=123456
tanya_vpn=au657
tanya_cache=f:\users\tanya\firefox
test=abcdef
fred=12345-12345-12345-12345

The vpn line I have just added. It won't work. I get no result. The batch code is

Code:

for /F "Usebackq Tokens=1* delims==" %%a in ("%BasePath%\%tfile%") do IF /I "%%a"=="%USERNAME%_vpn" set VPNserv=%%b

%tfile% = Tokens.txt.
%basepath%=I:\PostInstall
It's clearly finding the file because it's cycling through each line of the file.

Yet, this line works;

Code:

for /F "Usebackq Tokens=1* delims==" %%a in ("%BasePath%\%tFile%") do IF /I "%%a"=="%USERNAME%" set uWord=%%b

as does this;

Code:

for /F "Usebackq Tokens=1* delims==" %%a in ("%BasePath%\%tFile%") do IF /I "%%a"=="%USERNAME%_cache" set MozCache=%%b

As I watch the execution of the script I see this line;

Code:

IF /I "Tanya_vpn" == "Tanya_vpn" set VPNserv=au657

Windows does not consider this a match and does not set the VPNServ variable.

I've deleted the line and retyped it, in case there was some obscure non-printable character in there. I've tried using a dash instead of an underscore, but the cache entry works. I've put it as the first line and the last line in the file. Nothing works.

It's bizarre. I can't figure out what's causing the problem.

That's the problem, it is seeing the matching condition. It just doesn't see Tanya_vpn=Tanya_vpn as being a match, and it's picking up the resultant value of au657, so it's reading the file properly.

As you can see from the original post, the comparitor is not empty - it contains my name. And the comparitee is there as well (sorry, making up new words).

This is not contained within an IF ( .. ) block, so it's not a delayedexpansion issue either.