Diablo 4 players are learning the hard way not to ignore Whimsyshire portals
The rainbow portal can vanish if you finish another activity first.
If a rainbow portal to Whimsyshire appears in Diablo 4 Season 15, you might want to stop whatever you are doing and go through it immediately.
It turns out that the portal does not really hang around for long, and the game does not exactly go out of its way to warn you about that. One person learned this the hard way after finding a Whimsyshire portal during a Helltide, deciding to finish another activity first, and returning to discover that the rainbow portal was basically... gone.
And judging by the replies, they are very much not alone.
Whimsyshire portals can spawn inside of the pop-up Waking Nightmare dungeons or after killing a Goblin, and you might be tempted to collect your loot or fight a few more enemies before entering it, but this can cost you the entire portal.
We saw reports of roughly 1 to 2 minutes before the Portal leaves again, and given how rare these are, you should definitely head in there directly when it spawns.
You Cannot Go Back to Town
Once you made it inside of Whimsyshire, you will get a ton of loot. Probably way more than you can carry. And this is where the second problem comes up. You cannot go back to town to empty it, because if you teleport out, you can no longer get back in.
So if your inventory ends up full, you will have to make some tough choices when it comes to loot.
It Is Not the Only Portal With the Same Problem
This seems to be part of a bigger issue with Season 15's random legacy portals.
The pop-up Dungeons that can randomly spawn in the open world do not necessarily behave like normal dungeon entrances either. These portals can disappear relatively quickly as well. They tend to spawn a lot when you are doing rifts in Helltide, and if you ignore them, they will be gone before your rift is completed.
These also have another issue, since once you enter them, you will be teleported back to the nearest town when you try to leave again. The 'leave dungeon" option will not fix that either.
Thankfully, missing one portal does not lock you out of any zone permanently. You will probably get multiple portals through the Season, and if you complete Rank 5 of your Seasonal Ranks, you get one free 
Liquid Rainbow that provides a direct way to Whimsyshire.
- Ghost of the Cow King
So missing your first portal is not the end of the world. It is just going to hurt a little when you remember that you saw a rainbow portal, decided to finish your Helltide first, and then watched your chance to murder angry unicorns disappear into the void.
So if Whimsyshire appears, just go in!