I've run into situations where I get internmittent overlays with SQL bindings, which is what I imagined was happening in the previous topic concerning this same issue. When that happens, I get rid of the bindings and handle the data with scripting. Although, we're only getting one row here, so I can't imagine any reason why there would be any latency issues.
This is happening because readblocking returns a list. This needs to be written like this:
system.tag.readBlocking(["[default]MLWinTags/localVars/currentStepNumber"])[0].value
Note the addition of the [0] index value