Lua: utils.format_json omits all fields from a table created by utils.parse_json('[]') without error indication

Important Information

mpv version 0.33.1 on Ubuntu 21.10 amd64

There is a bug in either utils.parse_json or utils.format_json that can cause the latter to return an empty array [] but no error code (nil), when utils.parse_json is used on a JSON structure containing an empty table that is later populated.

Making a deep copy of the utils.parse_json return value works around the problem. So it appears to be some hidden property of the table returned by utils.parse_json that causes the resulting data loss in utils.format_json.

I understand that empty tables are a touchy issue in Lua vs. JSON, but utils.format_json should at least return an error instead of silently omitting all table entries if utils.parse_json can't be fixed to return "normal" empty tables from [].

I use JSON to store a "database" for some metadata values and noticed some fields would just disappear on saving until I learned to make a deep copy of the structure before conversion to JSON. Within Lua itself the table works normally and the fields keep their values however.

Reproduction steps

Minimal test.lua script, first converting a structure with empty table to JSON, then reconstructing and populating it but it remains empty with utils.format_json:

local utils=require 'mp.utils'

local tbl={data={}}
local json,err=utils.format_json(tbl)
print(json,err)
-- OK? '{"data":[]}',nil

tbl=utils.parse_json(json)
tbl.data["field"]=123
print(utils.to_string(tbl))
-- OK, field is set: '{"data" = {"field" = 123}}'

print(utils.format_json(tbl))
-- WRONG: '{"data":[]}',nil
-- Expected: either '{"data":{"field":123}}',nil or nil,'error'

Expected behavior

utils.format_json should either return the correct JSON string with all table entries, or an error, so that JSON can be used to store and recover Lua tables.

Actual behavior

utils.format_json returns an empty array [] for the table, but the error returned is nil.

Log file

[test] {"data":[]} nil 
[test] {"data" = {"field" = 123}} 
[test] {"data":[]} nil 

Sample files

see above

The report's test.lua reproduction is the starting point: inspect the utils.parse_json and utils.format_json entry points and run it on mpv 0.33.1 on Ubuntu 21.10. Done means populated data is encoded as an object, or format_json returns a non-nil error instead of silently returning [].