Multi-shard modification queries rejects IMMUTABLE function with column references

When trying to run
```
SELECT master_modify_multiple_shards('UPDATE geonames.geonames
SET geometry = ST_PointFromText(''POINT('' || longitude || '' '' || latitude || '')''
');
```
Citus returns with
```
ERROR: STABLE functions used in UPDATE queries cannot be called with column references
```

The problem is [ST_PointFromText](https://github.com/postgis/postgis/blob/87ed234dd5e5298462b7e352d796ac6d132d4465/postgis/postgis.sql.in#L4679-L4686) is defined as an IMMUTABLE function, so shouldn't this work? Or what is the STABLE function that is causing the error? Any functions used by `ST_PointFromText()` appear to be IMMUTABLE as well.