Is there a size limit on the URL encoding Algorithm?

We are using the url encoding algorithm to encode a uniface list to be passed as a command line parameter on starting another uniface session. 

This appears to occasionally truncate. The spawn command in use says it can take 8K as a command line, and it's definitely not that big. 

Is there some other limit I'm missing? Sizes of the parameters on the uniface command line? (Which are picked up in $1, $2 in the aps). 

Or, more likely, in the URL encoding algorithm, as per the question. 

Regards, 

Iain