The order in which tagged functions are invoked is not consistent across reloads. The game respects neither the order in which datapacks are enabled nor the order of functions within the tag's definition.

Expected results

The expectation is that the invocation order remains consistent across reloads:

[Server] foo:load1
[Server] foo:load2
[Server] foo:load3
[Server] bar:load1
[Server] bar:load2
[Server] bar:load3

See also dinnerquote.png, which is meant to serve as an alternate explanation of the expected behaviour and not an official piece of evidence.

Actual results

As evident in reload.png, the tag order is shuffled arbitrarily each time datapacks are reloaded. One iteration yielded the following output:

[Server] foo:load3
[Server] bar:load3
[Server] foo:load1
[Server] foo:load2
[Server] bar:load1
[Server] bar:load2

Details

The following also produce inconsistent results across reloads:

/function #foo:load
/function #bar:load
/function #foo:tick
/function #bar:tick

Given that foo and bar are defined in two separate datapacks, we can deduce two problems:

1. Datapack ordering is not respected; otherwise all invocations of foo functions should come before those of bar.
2. Function order within the tag definition is not respected; otherwise foo:load1 should be invoked before foo:load2, which in turn should be invoked before foo:load3.

How to reproduce

This behaviour can be reproduced by placing both datapacks from the attached datapacks.zip into a world's datapacks folder and running /reload repeatedly. (Datapacks are also available on GitHub.)

Code Analysis

Code analysis by liach can be found in this comment