Unexpected reset of read_only_allow_delete

In some of my indices, I'm doing "index.blocks.read_only_allow_delete": true by using the PUT /index/_settings API call. But after around 10 seconds, the setting disappears and the index is writable again.

I'm wondering if this can be a bug in ES, as in version 6.8 a change was made to reset this setting automatically when a node whose disk had gone over the flooding stage, was again below the normal thresholds.

I'm experiencing that odd behaviour in ES 7.9. What I expected is that, if ES changed the setting to true because of the watermarks, then it could reset it to false later. But if an operator changes the setting to true manually, then ES was going to respect that setting.

These are the docs where I read about that behaviour:

Controls the flood stage watermark, which defaults to 95%. Elasticsearch enforces a read-only index block ( index.blocks.read_only_allow_delete ) on every index that has one or more shards allocated on the node, and that has at least one disk exceeding the flood stage. This setting is a last resort to prevent nodes from running out of disk space. The index block is automatically released when the disk utilization falls below the high watermark.