Drowned have vastly different weights in rivers and frozen rivers

"In addition, this is different from the standard weight of 5 in (most) ocean type biomes."
This part is apparently intended though, to combat the issue of there being significantly less spawning spaces in rivers, compared to oceans.
 

Can confirm in 1.18 Pre-release 4.

Can confirm in 1.21.4, frozen_river.json contains the following

{
        "type": "minecraft:drowned",
        "maxCount": 1,
        "minCount": 1,
        "weight": 1
      }

Compared to river.json which contains

{
        "type": "minecraft:drowned",
        "maxCount": 1,
        "minCount": 1,
        "weight": 100
      }

To fix this, the drowned spawner object can be copied from river.json or simply changed from 1 to 100.