Lens graph formula missing fit(carry) ability in a formula summing up multiple indexes with sparse data

In timelion, if I had 2 indexes of data:
a with 1 data point every day at midnight
b with 1 data point every hour

I could create a graph with hourly intervals, and then sum the 2 indexes together in a single line that would be the sum of the 2 values every hour over the course of a day - which would increment up every hour:

.es(index=a,...).fit(carry).add(.es(index=b,...))

But, in the lens graph, if I wanted to create the same line with a fomula, the hourly increments would not show up in the line plotted until the next doc in b index occurs, i.e. the line would only increase in value every 24 hours despite a increasing in value every hour.

cumulative_sum(sum('field_b', kql='_index : "b"'))
+
last_value(field_a, kql='_index : "a"')

This issue might well be the crux of it - [Lens] Formula: provide fitting function for missing/null values · Issue #115761 · elastic/kibana · GitHub - does anyone have a workaround in the interim? Running Kibana 7.15.2

Thanks in advance!