These are some considerations:
1. In my opinion, I have personally looked at tens of thousands of layout plans in my life-time. And maybe blame it on falling memories.. the most distinct are some of the layout plans of Regent Grove condo at Choa Chu Kang. Even if with a WC, beside the main door there is a solid wall between the WC and the main entrance.
2. The other are some landed properties in the past where the developer had placed a "powder room" (many such powder rooms have a WC). Frankly, there are more of such incidences related to landed property. But your question is not about landed properties. It is about apartments.
3. So far as I can recall, I seldom if ever see any HDB flats that has a toilet next to the front door. And yes, the government (HDB statutory board) had built close to 98% of the total housing in Singapore. And yet, correct me if I am wrong; mostly are kitchens but I can't recall any toilets next to the main entrance of such door.
4. What I am trying to get to is that even if there is no major issues in Feng Shui; BUT under the Holy Trinity of Chinese luck system:
4.1. Heaven luck or born with = Ba Zi = 33.33%
4.2. Earth luck = Feng Shui = 33.33%
4.3 Human or man luck: Feeling good or bad etc.. = 33.33%
Total = 100%
Here, another consideration is the "Human factor".
5. For example, if today, I say, it is still acceptable under Shapes and Forms provided, the exernal Shapes and Forms are okay and there are few "major leaks" in the home.
5.1. Thus even if this toilet passes under Feng Shui; but if someone has bought this house and STILL in doubt or whenever visitors visit the person would comment : "You know.. it is bad luck to have a toilet at and seen from the main door".
5.2. This Human or man luck = feeling good or bad may eventually bring down the Human Luck factor. And remember it also accounts for 33.33% share of one's total luck.
6. Seriously, as mentioned under Para 1, in my opinion, I have literally seen tens of thousands of layout plans in my life-time and maybe, just maybe such layouts account for a very small percentage of this total.
7. In the future, if someone were to bring in a regular geomancer or a very conservative geomancer to help him/her assess the unit before the potential owner buys it. The odds are that many geomancers will see or use this as a RED Flag. And may even tell the potential buyer.. go find a better unit.