data association converts date + duration expression to a string

Summary

data association converts date + duration expression to a string

Content

While the simple expression builder considers the following date arithmetic expression to be valid, the data association window shows that the express is being converted to a string and thereby considers it invalid:

date('now') + duration("PT"+hoursDO+"H")

Meanwhile, the data association window is perfectly happy with these:

'now' + 'PT1H'
date('now') + duration('PT1H')

Some screen shots to add context...

I have come up with a not very desirable workaround, but I'd like to be able to do this in a single expression like above.  Any help/thoughts appreciated!