I would ask the landscape designer as it was their suggestion to lay the mulch first, and they are familiar with the site.

Yes, will do!

Be ready to be constantly out with a broom sweeping the mulch off the stones. Birds love tossing mulch about.

Usually, flagstones are put in sand. You dig down several inches, level, put in sand, which you level and tamp and then lay your flagstones.

Sand can wash out in heavy rains. Use stone dust for the base & filler between stones.

Sand, stone dust, traffic bond…I’ve used them all without issue. Just depends on the lay of the land.

tj

I'm not sure I'd go back to the landscape designer for help. One of the fundamental rules of a landscape installation is that ALL hardscaping goes in before any planting or 'finishing' touches (like mulching) are done. And a flagstone pathway that is expected to perform as intended and last should be set into a properly prepared base. Which includes an edging restraint to contain all base material for those of you who have never laid one.

Thank you, everyone!

If the landscape designer didn't do the job properly, he/she needs to be held to account.

The designer designs...they are not responsible for any installation (unless part of the design package) so how they can be held "accountable" is somewhat moot. The only error was the misinformation about sequencing. But since this seems to be a DIY project, even that is easily recoverable.

I understand why the designer instructed you to do the compost and mulch first provided the compost was mixed with the underlying soil and then the mulch thrown on top.... This does not mean to simply place the flagstones on top of the mulch, it just means to apply the mulch compost first because placing the flagstone first would make it harder to mix in the compost...So its not necessarily a mistake by the designer. In any case, the flagstones should be set on a fine gravel or sand base.