Thais,
Firstly, my initial impression - without checking your profile - was that you are an experienced Airbnb Host, in which case it seemed odd that you would chose to change your flexible Check-In time based on one experience.
However, your profile shows only 2 reviews from September & October 2018. Perhaps you're operating on a new account? Even so it's 3 months since a last booking on this profile.
Curiousity aside, based on 6 years of experience of Flexible Check in & out times I wouldn't change it for any other. In my case, because it's most suitable, I predominantly work from home, and often late in the night; so hardly ever perturbed if Guests don't turn up on time. Indeed, I've had Guests turn up at every hour round the clock. It's part of the nature of travel, delays unavoidable.
In all this time I only recall two occasions as you describe. Far more often it's rather 24 hours later, i.e. in the early hours of the 2nd day. -- In any case, Guests have always been most grateful that they can comfortably arrive without undue worry or feeling of inconveniencing the Host.
Last but not least, the Flexible option can be also beneficial for me. On a few occasions I knew I couldn't be home at e.t.a. and kindly asked the guests to turn up later. Though more common is that room may not be ready, but Guests are welcome to drop off luggage and pick up keys, so return at an agreed time when the room will be ready.
In sum, my advice, if it generally works, don't fix it, because no doubt sooner or later you will experience a problem with a fixed check-in. It's swings and roundabouts, and ultimately what's most suitable in the normal course of events for you.
