You did the session you always do, felt worse, and produced twenty watts less. Almost every rider produces less power indoors than out, and the reasons are well understood.
Heat, mostly
Outdoors you ride through moving air, and even at a modest speed there is a constant breeze stripping heat from your skin, so your body spends very little effort on cooling.
Indoors there is no airflow except what you create. Core temperature rises, blood is diverted to the skin to shed heat, plasma volume falls as you sweat, and your heart rate climbs to move the same oxygen. Eventually your body declines to keep producing the power you are asking for.
The size of the effect is individual, but a gap of roughly five to ten per cent between outdoor power and poorly cooled indoor power is common, and riders who sweat heavily or train in a warm room see more.
Position and the absence of freewheeling
Two smaller effects sit behind heat.
Indoors the bike does not move. Outdoors you shift constantly, stand on climbs, rock the bike and recruit slightly different muscle at every change, whereas on a fixed trainer you sit in one position and load the same fibres for the whole session, which fatigues them faster.
And you never stop pedalling. An outdoor hour contains junctions, descents, corners and a surprising amount of coasting, while an indoor hour is an hour of pedalling, so the ride is denser and feels harder for the same average number.
What this means for your FTP
There are two schools of thought here, and one is more practical than the other.
Keeping two figures is more precise, an outdoor threshold and an indoor one, each used for the rides it fits. Riders with a lot of data tend to end up here.
Keeping one, set indoors is what we would suggest if you train mainly indoors over winter. Test in the conditions you actually train in and your zones will be right for the sessions you do most, so your outdoor rides will feel slightly easy by comparison, which is a far better failure mode than the reverse.
The mistake to avoid is testing outdoors in September, training indoors all winter against that number, and concluding after six weeks of failed intervals that you are going backwards. Your zones are simply set for a different environment.
WattPlan works from one FTP figure, so if you are about to move indoors for the season it is worth re-testing in the room you will be training in. Every workout is then scaled to a number that matches the conditions.
What does not explain the gap
Your trainer is probably not lying to you in any way that matters week to week. A trainer that reads consistently low is still perfectly useful for training, because every session is scaled against the same reference. Absolute accuracy matters when you compare your numbers with someone else's, and repeatability is what matters for training, which most trainers manage.
It is not lost fitness. Fitness does not fall between a Saturday outdoors and a Tuesday indoors.
It is not that you have gone soft. Heat is a physiological constraint rather than a character flaw, so a bigger fan will do more for you than trying harder.
The practical version
Get the biggest fan whose noise you can tolerate and put it where it hits your chest and face. Start with the room as cool as you can stand, because you will be grateful twenty minutes in, and drink more than feels necessary. Test indoors if you train indoors, and compare indoor rides with other indoor rides rather than with that day in August when it was 18 degrees and you had a tailwind.