原因

当UICollectionViewCell的size不为整数时，UICollectionViewFlowLayout在布局计算时，可能会调整Cell的frame.origin，使Cell按照最小物理像素（渲染像素）对齐，导致出现缝隙。
假设当前Cell的frame.origin.y=100.8（逻辑像素），转化成渲染像素（参考备注）是201.6（iPhone 8）或302.4（iPhone 8 Plus）。为了按渲染像素对齐，UICollectionViewFlowLayout应该会四舍五入取整，取整后为202（iPhone 8）或302（iPhone 8 Plus），转成逻辑像素101（iPhone 8）或100.667（iPhone 8 Plus），导致在iphone8上就会出现0.2像素的缝隙。
分辨率相关的，可以百度下。

简单解决办法：

主动把Cell的size取整，不丢给UICollectionViewFlowLayout处理。

- (CGSize)fixedCollectionCellSize:(CGSize)size {
    CGFloat scale = [UIScreen mainScreen].scale;
    return CGSizeMake(round(scale * size.width) / scale, round(scale * size.height) / scale);
}

Demo实验

UITableView，Cell高度设置成100.12，没有强制被按渲染像素对齐，如 99.62 1802.16 1902.28

(lldb) po [0x7fb85b83b800 recursiveDescription]
<UITableView: 0x7fb85b83b800; frame = (0 0; 375 667); clipsToBounds = YES; gestureRecognizers = <NSArray: 0x6000004451f0>; layer = <CALayer: 0x60000022dc40>; contentOffset: {0, 1272.5}; contentSize: {375, 2002.4000549316406}; adjustedContentInset: {20, 0, 0, 0}>
   | <UITableViewCell: 0x7fb85c862a00; frame = (0 1902.28; 375 100.12); autoresize = W; layer = <CALayer: 0x6040004265a0>>
   |    | <UITableViewCellContentView: 0x7fb85b70fea0; frame = (0 0; 375 99.62); gestureRecognizers = <NSArray: 0x6040002426a0>; layer = <CALayer: 0x604000425e80>>
   |    |    | <UIImageView: 0x7fb85b41a330; frame = (0 0; 0 0); userInteractionEnabled = NO; layer = <CALayer: 0x6000008215c0>>
   |    | <_UITableViewCellSeparatorView: 0x7fb85b710820; frame = (15 99.62; 360 0.5); layer = <CALayer: 0x604000427640>>
   | <UITableViewCell: 0x7fb85c862400; frame = (0 1802.16; 375 100.12); autoresize = W; layer = <CALayer: 0x6040002212e0>>

iPhone Plus 8，UICollectionView，Cell高度设置成100.12，强制被按渲染像素对齐了，frame.origin.y被调整后的值，如200.333 500.667 600.667

   | <UICollectionViewCell: 0x7fdd9f71f010; frame = (0 200.333; 414 100.12); hidden = YES; layer = <CALayer: 0x60000023d800>>
   | <UICollectionViewCell: 0x7fdd9f71f470; frame = (0 300.333; 414 100.12); layer = <CALayer: 0x60000023e380>>
   | <UICollectionViewCell: 0x7fdd9f72aa30; frame = (0 400.333; 414 100.12); layer = <CALayer: 0x60000023f4e0>>
   | <UICollectionViewCell: 0x7fdd9f72ae90; frame = (0 500.667; 414 100.12); layer = <CALayer: 0x60000023f9a0>>
   | <UICollectionViewCell: 0x7fdd9f72b510; frame = (0 600.667; 414 100.12); layer = <CALayer: 0x60000023fbc0>>
   | <UICollectionViewCell: 0x7fdd9f72d1a0; frame = (0 701; 414 100.12); layer = <CALayer: 0x60000023fdc0>>

iPhone Plus，UICollectionView，Cell高度设置成100.12，强制被按渲染像素对齐了，frame.origin.y被调整后的值，如300.5 400.5 500.5

   | <UICollectionViewCell: 0x7f85b441b6c0; frame = (0 0; 375 100.12); hidden = YES; layer = <CALayer: 0x600000035ec0>>
   | <UICollectionViewCell: 0x7f85b4502e10; frame = (0 100; 375 100.12); hidden = YES; layer = <CALayer: 0x600000035c20>>
   | <UICollectionViewCell: 0x7f85b4505c10; frame = (0 200; 375 100.12); layer = <CALayer: 0x600000036300>>
   | <UICollectionViewCell: 0x7f85b4506400; frame = (0 300.5; 375 100.12); layer = <CALayer: 0x600000036be0>>
   | <UICollectionViewCell: 0x7f85b4506d70; frame = (0 400.5; 375 100.12); layer = <CALayer: 0x600000227ac0>>
   | <UICollectionViewCell: 0x7f85b4507560; frame = (0 500.5; 375 100.12); layer = <CALayer: 0x6000002277c0>>